Larger body size in cold climates? Fundamentals of Bergmann’s Rule

Nature, as we well know, is not an inert or homogeneous medium, but rather it produces a multitude of phenomena and processes that happen every second that passes, giving rise to important environmental transformations. It is a living environment, with different habitats and environments, where an endless number of physical, chemical and, of course, biological interactions originate daily, creating an environment full of different shades and tonalities. In this way, it can be a complex task to discover laws that accurately determine the behavior and functioning of nature (and its inhabitants).

It is, therefore, logical to think of the fact that the different possible populations of the millions of species (and subspecies) that inhabit the Earth will not obey constant patterns and norms, but there will be significant variations between them depending on the environment they inhabit. In addition, in the different populations of the same species, we may find interesting differences in characters. These variations may refer both to behavioral patterns (animal behavior) and to physical features and alterations in external appearance, easily observable thanks to the phenotype: for example, different kinds of pigmentation, coloring patterns, types of claim, body weight, etc.

In this article, we will focus on one of these differential traits: body size. Thus, we will explain the fundamentals that intervene to establish size variations in different populations and individuals of the same species, and we will clarify some considerations.

Bergmann’s rule for understanding differences in body size
Carl Bergmann (1814-1865) was a German naturalist and biologist who, in 1847, developed one of the most important biogeographical rules in biology and ecology: the well-known Bergmann rule. In it, Bergmann established some guidelines related to the body mass of certain animals (his studies focused on endothermic mammals and birds), reaching the following conclusion: there was a tendency to increase the average body size of populations of a species, or species within a taxon, by increasing latitude and altitude. In other words, the largest body sizes were found in the populations furthest from the equator and on the summits.

Initially, this trend was attributed to a decrease in the relationship between surface and volume (S/V) as size increased, a presumably advantageous fact that would allow a greater amount of body heat to be conserved in colder climates, preventing its dissipation. Thus, in terms of thermoregulation, it would be beneficial to have a large size in regions with low temperatures. However, these assertions are not entirely categorical, since over the years a series of doubts and criticisms have been expressed towards the fundamentals and bases of this rule, which we reveal below: